High intensity discharge (“HID”) lamps are typically used when high levels of light are needed over large areas such as gymnasiums, warehouses, parking lots etc. The HID lamps provide high lumen output and high-energy efficiency. Within the automotive industry, HID lamps are replacing conventional incandescent halogen lights used for automotive headlamps. In an HID lamp, light is generated by means of an electric discharge that takes place between two metal electrodes enclosed within a sealed envelope or arc tube. At least with respect to automotive headlamps, the arc tube is composed of quartz, because quartz has a relatively high in-line transmission of light at wavelengths within the visible light spectrum.
The discharge medium in quartz metal halide lamps consist of a mixture of xenon, mercury, sodium iodide (NaI) and/or scandium iodide (ScI3), wherein the surrounding envelope, or arc-tube, is made of quartz with tungsten electrodes protruding within the envelope. In operation, the lamp size is kept small enough for optical coupling purposes. Further, the lamps are required to meet the automotive industry standard of starting fast by delivering at least eighty percent of their steady state lumens no later than four seconds from the point at which they are turned on. The small lamp size and fast start requirements result in higher wall thermal loading, which in turn poses some limits on the quartz envelope material, and significant thermal stresses in the arc-tube, especially near the electrode roots. These limitations result in shortening the lamp life and also decreasing reliability of the lamp.
Ceramic materials, such as polycrystalline alumina (PCA) and yttrium aluminum garnet (YAG) are some times used as an envelope material in HID lamps. Ceramic arc tubes can withstand higher temperatures; and, the cold spot temperature in ceramic lamps can be driven to a high enough value to evaporate the metal halide dose and produce enough vapor pressure for both the light emitting elements and the buffer gas. However, these ceramic materials are not as transparent, or translucent as quartz, and have lower in-line transmission of light than quartz. The highest reported inline transmission of quartz is 91%, while the maximum theoretical inline transmission for YAG is 83%. This is due to the difference in refractive index of the two materials, with quartz having a lower value than YAG (1.52 vs. 1.84). The YAG ceramic material has a higher in-line transmission than the PCA, because of its cubic crystal structure; however, YAG ceramic arc tubes used in general lighting applications, such as high-pressure sodium lamps, have an in-line transmission less than 50%. Such transparency is not suitable for HID lamps used in optical applications such automotive headlamps. Previously developed YAG arc tubes were also too expensive to compete with PCA, even though they had superior corrosion resistance against high-pressure sodium dose chemistries.
Arc tubes are produced and sold that are composed of a YAG ceramic material that have a reported in line transmission over 80%; however, such arc tubes are less suitable for use with optical applications such as automotive headlamps. These arc tubes are fabricated using a molding or shaping process known as slip casting. The slip casting process is limited to the production of the arc tube shape including an oval shape of the arc body as shown in FIG. 1. This configuration limits the flexibility of where the electrode tip is positioned with the arc tube chamber. That is, the sloped configuration of the neck portion positions the neck portion close to the electrode. In a ceramic arc tube shown in FIG. 1, the highest stress point on the arc tube wall is located at the center of the chamber wall. This stress, which is caused by thermal gradients, can be managed by carefully choosing the right dimensions (inside diameter, thickness and length) of the chamber. An additional high stress point is found in the neck portion of the arc tube. The location and stress value of this additional stress point are very sensitive to the position of the tungsten electrode tip with respect to the neck portion. This stress point imposes a further limitation in the design of the arc tube, and compromises the life and performance of the lamp leading to a less robust design.
Other shapes have been used for arc tubes such as the box-like configuration shown in FIG. 2, which is used for general lighting applications and not automotive headlamps. As compared to arc tube chambers for automotive headlamps, the arc tube chamber is larger, typically greater than 5 mm, and the wall thermal load is much lower at 40 watts/cm2. The arc tube chamber for an automotive headlamp is 2 mm or smaller and has a wall thermal load of about 120 watts/cm2. Such a configuration has been used in arc tubes composed of polycrystalline alumina; however, it has not been used to produce YAG arc tubes.
In a paper entitled Precipitation and Calcination Processes for Yttrium Aluminum Garnet Precursors Synthesized by the Urea Method, J. Am. Ceram. Soc., 82[8], 1977-1984 (1999), there is disclosed a method for making YAG using a wet chemistry that generally includes the addition of yttrium chloride and aluminum chloride in a given concentration of ammonium sulfate and urea. This solution is stirred at 95° C., and upon cooling, a YAG precipitate is formed that can be rinsed and calcined. Powders produced through wet chemistry processes are more expensive than engineering grade alumina powders, which corresponds to a higher price for arc tubes produced using this method over PCA.
YAG may be created in a solid-state reaction, in which an yttrium oxide powder is mixed with aluminum oxide powder and ethyl silicate as disclosed in the article Fabrication of Polycrystalline, Transparent YAG Ceramics by a Solid State Reaction Method. J. Am. Ceram. Soc., 78 (1) 225-228 (1995). A powder mixture of yttrium oxide and aluminum oxide was milled for a predetermined amount of time and dried to powder form. The powder mixture was then isostatically pressed to form ceramic discs. The discs were sintered and the YAG was formed during the sintering process. However, it is uncertain whether such a powder mixture can be manipulated to form more complex shapes as through extrusion or injection molding, because the shaped discs were formed through isostatic pressing. The Y2O3 and Al2O3 powders used to form YAG were synthesized via alkoxide precipitation and pyrolysis. Such powders may have a high surface area that would limit their use in forming processes such as extrusion or injection molding and will also be more costly than engineering grade powders. The grain size (about 10 μm) achieved using this method is larger in diameter than desirable for certain applications such as automotive headlamps, where a smaller grain size (less than 5 μm) and an associated higher mechanical strength are required.
Another solid-state reaction to form YAG is disclosed in U.S. Publication No. 2006/0100088 A1 for a transparent multi-cation ceramic. The method disclosed in this publication uses nanopowders having particle sizes of less than 1 micron, and preferably below 500 nm. The YAG ceramics described in this published application were observed to have an inline transmission of only 65%. The publication does not disclose results achieving 80% in-line transmission. Moreover, nanopowders are expensive, increasing the cost of production of ceramic components. In addition, nanopowder mixtures have greater particle surface area and require more water to hydrate the powders to create a moldable mass. Such a mass is difficult to use in extrusion or injection molding processes.